r o b e r t s o n e n v i r o n m e n t a l MONITORING | MANAGEMENT | RESEARCH Taranaki Regional Estuaries Ecological Vulnerability Assessment For Taranaki Regional Council July 2019 REPORT INFORMATION & QUALITY CONTROL Prepared for: Taranaki Regional Council C/- Thomas McElroy, Environmental Scientist - Marine Biology Authors: Dr Ben Robertson Principal Consultant, Director Internal Reviewer: Dr Barry Robertson Technical Advisor, Director Document Name: TRC2019EstuaryVA_FINAL_v1 (22 August 2019) Version History: TRC2019EstuaryVA_DRAFT_v0.5 (26 July 2019) Ben Robertson (Principal Consultant, Director) Barry Robertson (Technical Advisor, Director) 151 Awa Awa Road BSc (Hons), PhD BSc, Dip Sci, PhD Tasman 7173 Jodie Robertson (Senior Consultant, GIS Tech) Julian Goulding (Technical Officer, Skipper) Phone: +64 27 823 8665 BSc, PG Dip, MSc BComm, Master 3000 Gross Tonnes robertsonenvironmental.co.nz Contents Executive Summary . 1 1 Introduction . 6 1.1 Project Brief and Scope . 6 1.2 Report Structure . 6 2 Assessment Methodology . 8 2.1 Vulnerability Assessment and Monitoring Recommendations . 8 2.2 Summary of Estuary Vulnerability Assessment Steps . 10 3 Results . 14 Mōhakatino Estuary . 14 Tongaporutu Estuary . 17 Mimi Estuary . 20 Urenui Estuary . 23 Onaero Estuary . 26 Waitara Estuary . 29 Waiongana Estuary . 32 Waiwhakaiho Estuary . 35 Te Henui Estuary . 38 Tapuae Estuary . 41 Oakura Estuary . 44 Timaru Estuary . 47 Katikara Estuary . 50 Kaupokonui Estuary . 53 Waingongoro Estuary . 56 Tangahoe Estuary . 59 Manawapou Estuary . 62 Patea Estuary . 65 Whenuakura Estuary . 68 Waitotora Estuary . 71 4 Summary . 74 5 Monitoring Recommendations . 76 6 References . 78 7 Limitations . 81 Phone: +64 27 823 8665 iii robertsonenvironmental.co.nz List of Appendices Appendix A: Major Issues facing NZ Estuaries . 82 Appendix B: Detailed Data Taranaki Regional Estuaries . 87 Appendix C: Estuary-Specific Vulnerability Matrices . 103 Appendix D: Broad Scale Habitat Classifications . 124 Appendix E: Field Photographs . 128 List of Tables Table 1. Main estuary categories used in susceptibility analysis . 13 Table 2. Summary of monitoring recommendations . 77 List of Figures Figure 1. Map of estuaries assessed, Taranaki Region, 2019 . 7 Figure 2. Summary vulnerability assessment steps . 9 Figure 3. Summary of broad scale habitat, Mōhakatino Estuary . 15 Figure 4. Summary of broad scale habitat, Tongaporutu Estuary . 18 Figure 5. Summary of broad scale habitat, Mimi Estuary . 21 Figure 6. Summary of broad scale habitat, Urenui Estuary . 24 Figure 7. Summary of broad scale habitat, Onaero Estuary . 27 Figure 8. Summary of broad scale habitat, Waitara Estuary . 30 Figure 9. Summary of broad scale habitat, Waiongana Estuary . 33 Figure 10. Summary of broad scale habitat, Waiwhakaiho Estuary . 36 Figure 11. Summary of broad scale habitat, Te Henui Estuary . 39 Figure 12. Summary of broad scale habitat, Tapuae Estuary . 42 Figure 13. Summary of broad scale habitat, Oakura Estuary . 45 Figure 14. Summary of broad scale habitat, Timaru Estuary . 48 Figure 15. Summary of broad scale habitat, Katikara Estuary . 51 Figure 16. Summary of broad scale habitat, Kaupokonui Estuary . 54 Figure 17. Summary of broad scale habitat, Waingongoro Estuary . 57 Figure 18. Summary of broad scale habitat, Tangahoe Estuary . 60 Figure 19. Summary of broad scale habitat, Manawapou Estuary . 63 Figure 20. Summary of broad scale habitat, Patea Estuary . 66 Figure 21. Summary of broad scale habitat, Whenuakura Estuary . 69 Figure 22. Summary of broad scale habitat, Waitotora Estuary . 72 iv Executive Summary Robertson Environmental Limited has been engaged by Taranaki Regional Council (TRC) to un- dertake the vulnerability assessment of twenty estuaries in the Taranaki Region in relation to the key coastal issues of eutrophication (excessive nutrients) and sedimentation (excessive muddi- ness), and to use the resulting information to inform long-term estuary monitoring recommenda- tions. The purpose of the assessment was to characterise each estuary’s current ecological condition in relation to eutrophication and sedimentation, and compare the findings with relevant national stan- dards (NZ Estuary Trophic Index, NZ ETI), to provide recommendations regarding future monitor- ing priorities at a regional scale. The fieldwork was undertaken in February 26th - 4th March 2019, and the results, overall vulnerability ratings, and monitoring recommendations are outlined below (see summary table on next page). Estuary Vulnerability to Eutrophication and Sedimentation As is characteristic of estuaries on the West Coast of NZ, all twenty of the Taranaki Region estuar- ies assessed were shallow, short residence time, tidal river estuaries (SSRTREs), each variable in size and partially separated from the sea by a range of physical features. The results showed that each estuary fits into one of four sub-types (based on physical attributes and freshwater inflows), each with different vulnerabilities to nutrients and fine sediment and therefore long-term monitoring requirements, as follows: Estuary Type 1. Short length, low flow SSRTREs - <1 km long, beach located, low freshwater inflows (<1 3m s-1), mouth sometimes restricted/closed. Taranaki Region estuaries that fit into this sub-group included Tapuae, Timaru, Te Henui, and Katikara Estuaries. • Physical characteristics: Very short length, often beach located SSRTREs consist of rela- tively narrow channels situated between the upper edge of the beach and the tidal level. In some situations the channel meanders along the back of the beach for a small distance before entering the sea, whereas in others the discharge path is more direct. A few expand into small lagoons around the upper high water area. In very high tides and storm surges, saline water enters the stream inland of the beach for a small distance. At times the mouth is often restricted and can sometimes close for short periods, during which time the upper beach lagoon may expand and show eutrophication/sedimentation symptoms. • Overall vulnerability: With the exception of Katikara Estuary, which was shown to be highly vulnerable to eutrophication impacts, Type 1 estuaries were the least vulnerable of the Ta- ranaki Region estuaries to eutrophication and sedimentation. The main reason for this was their small size, comparatively low ecological diversity, and regular periods of high flushing (even though some examples experience periodic mouth closure/restriction). Consequently, although estimated nutrient and sediment loads to the estuaries were generally large, they are unlikely to be subjected to prolonged periods of eutrophication and muddiness. Synop- tic surveys of this estuary type in Feb/March 2019 confirmed the absence of symptoms of eutrophication (i.e. opportunistic macroalgal and/or phytoplankton blooms) or sedimentation (extensive areas of soft muddy sediments), while Katikara Estuary had phytoplankton issues as indicated by highly elevated chlorophyll a concentrations throughout the subtidal channel habitat. Estuary Type 2. Moderate length, low flow SSRTREs - 1-3 km long, low freshwater inflows (<2 m3 s-1), mouth sometimes restricted/closed. Taranaki Region estuaries that fit into this sub-group included Waiongana, Mimi, Manawapou, Onaero, Waingongoro, Kaupokonui, and Oakura Estu- aries. • Physical characteristics: Moderate length SSRTREs consist of relatively narrow chan- nels situated between the tidal level and approximately 1-3 km inland. In some situations the channel meanders along the back of the beach for a distance before entering the sea, whereas in others the discharge path is more direct. A few expand into small lagoons around the upper high water area. The estuary mouth is generally open to the sea but in others it is often closed (e.g. Onaero Estuary). 1 Summary of NZ ETI-based susceptibility, current condition and overall vulnerability ratings, and monitoring recommendations, for twenty Tarana- ki Region estuaries, 2019. * See further details in ‘Estuary Monitoring Recommendations’ below. Coastal Stressor Overall Sub- Vulner- Recommended Monitoring Estuary Sedimentation Eutrophication Type1 ability Monitoring* Frequency Current Current Suscepti- Suscepti- Condition Condition bility bility (2019) (2019) Tapuae Moderate Moderate Minimal Minimal Moderate Timaru Moderate Moderate Minimal Minimal Moderate Synoptic monitoring only 10-yearly Te Henui Moderate Moderate Minimal Minimal Moderate SSRTRE Type 1 SSRTRE Type Katikara Moderate Moderate Moderate High Mod-High Eutrophication-targeted monitoring Annually Waiongana Moderate Moderate Minimal Minimal Moderate Synoptic monitoring only 10-yearly Mimi Mod-High Very High Very High Moderate High Broad- & fine-scale monitoring 3-year baseline, 5-yearly Manawapou Moderate Moderate Minimal Minimal Moderate Onaero Moderate Moderate Minimal Moderate Moderate Synoptic monitoring only 10-yearly Waingongoro Moderate Minimal Minimal Minimal Minimal SSRTRE Type 2 SSRTRE Type Kaupokonui Moderate Moderate Minimal Minimal Moderate Oakura Moderate Moderate Moderate High Mod-High Eutrophication-targeted monitoring Annually Tangahoe Moderate Moderate Minimal Minimal Moderate Synoptic monitoring only 10-yearly Urenui Mod-High Very High Very High Moderate High Type 3 SSRTRE Mōhakatino Mod-High Very High Moderate Moderate High Waitotara Mod-High Very High Minimal Minimal Mod-High Broad- & fine-scale monitoring 3-year baseline, 5-yearly Waitara Mod-High Very High Minimal Moderate Mod-High Patea Mod-High Very High Very High